1. Field of the Invention
The present invention relates to a method for a novel electric field orientation of liquid crystal polymers and oriented materials of liquid crystal polymers, obtained by the method. The polarized high molecular liquid crystal materials obtainable by the method of the invention can be utilized as piezoelectric materials, pyroelectric materials, electret materials, non-linear optical elements and so on.
2. Brief Description of the Prior Art
Liquid crystal polymers capable of forming an optically anisotropic melt phase, which are known commonly as thermotropic liquid crystal polymers, are roughly grouped into a class of main chain liquid crystal polymers in which a rigid molecule such as a benzene ring, biphenyl ring or naphthalene ring constitutes the main chain skeleton and a class of side chain liquid crystal polymers in which a liquid crystal-forming molecule called `mesogen` is bound to a flexible skeletal polymer through a spacer group. The side chain liquid crystal polymers are such that the liquid crystal properties of monomeric or low molecular liquid crystals have been supplemented with the properties of polymers and, as such, promise usage as various functional materials. Thus, a side chain liquid crystal polymer in molten state undergoes change in orientation upon application of an external field such as an electric field or a magnetic field and retains the acquired orientation when its temperature is decreased with the application of the external field being continued. Therefore, the possibilities of utilizing such compounds as information storages, temperature indicating materials and so on have been suggested. For example, it is described in Japanese Laid-open Patent Publication No. 60-114823 (1985) and U.S. Pat. No. 4,702,558 that a side chain liquid crystal polymer having a siloxane backbone undergoes change in the state of orientation upon application of a direct or alternating electric field in the neighborhood of the transition temperature from the optically anisotropic melt phase to the isotropic melt phase and that since particularly a unidirectionally oriented optically transparent state can be achieved by applying an alternating electric field, this material can be applied to information storage devices. It is shown in Die Makromolekulare Chemie Rapid Communications 2, 305, (1981) and Polymer Communications 24, 364, (1983) that Shibaev et al. obtained a uniformly oriented "homeotropic" transparent film by applying an alternating electric field to a side chain liquid crystal polymer having a polymethacrylate backbone and that information can be recorded in the film by local heating of the film by means of, say, a laser beam, so as to disturb the state of orientation.
Furthermore, it is mentioned in Japanese Laid-open Patent Publication No. 61-69039 (1986) and U.S. Pat. No. 4,624,872 that when an exclusively aromatic thermotropic liquid crystal polymer film is exposed to a direct electric field, the liquid crystal molecules are oriented perpendicularly with respect to the film plane, thus permitting use of the film as a non-linear optical element.
It is, thus, known that as a DC or AC field is applied to a thermotropic liquid crystal polymer film, the liquid crystal molecules undergo movement to induce a change in the state of their orientation. However, while an optically transparent homeotropic film comprising liquid crystal molecules aligned in one direction can be obtained by application of an AC field, the resulting oriented film is also oriented about the center of symmetry and not polarized, apparently because of the electrical symmetry of the AC field. For example, according to the investigation conducted by the present inventors, the oriented film obtained by subjecting a 20 .mu.m-thick film of a polymer of the following formula (I) to an AC field at a frequency of 3.5 KHz and 500 Vrms at a temperature of 90.degree. C. and quenching it to room temperature without removal of the AC field was transparent and did not transmit light at all when observed under a polarizing microscope using a crossed Nicol prism. ##STR1##
However, when the surface charge of this oriented film was measured with a static meter, the film surface was found to have not been charged at all. No generation of an electric potential was found, either, when the piezoelectric property of the film was estimated by applying a certain impact to the film and measuring the generated voltage in accordance with the method described hereinafter in the working examples. These findings also suggest that the side chain moieties of the oriented film obtained by application of an AC field are disposed about the center of symmetry. This symmetric orientation is sufficient and useful for information recording applications such that the homeotropic oriented film is locally heated by a laser emission or the like to induce a change in orientation and this change is read from the resultant change in the reflection of light or for such applications as temperature sensors in which temperature-dependent optical changes in the state of orientation are utilized. However, such a non-polarized oriented film cannot be expected to be applied to the another polyfunctional materials in, for example, piezoelectric or pyroelectric materials.
On the other hand, there remains the possibility of obtaining a polarized oriented film by the application of a DC field. However, it is known that liquid crystal polymers and particularly molecules of a side chain liquid crystal polymer generally can be provoked to move but be hardly oriented uniformly by application of a DC field. It has been reported in the above-cited Japanese Laid-open Patent Publication No. 60-114823 (1985), U.S. Pat. No. 4,702,558, and Polymer 26, 1801 (1985), for instance, that when a DC field or an AC field of 300 Hz or less is applied to a side chain polymeric liquid crystal compound having a siloxane backbone, a turbulence is generated so that a uniformly oriented film cannot be obtained. According to the present inventors' exploration in which a 20 .mu.m-thick film of a liquid crystal polymer of the above formula (I) is subjected to a DC field at a voltage up to 2000 V at 90.degree. C., a turbulence was generated attesting to the movement of liquid crystal molecules but it was not possible to obtain an optically transparent film of uniformly oriented liquid crystal molecules.
Die Makromolekulare Chemie 183, 1245 (1982) also mentions that liquid crystal polymers are not uniformly oriented by a DC field or a low-frequency AC field.
Thus, by any of the hitherto-reported methods for orienting liquid crystal polymers with the use of an electric field, it is impossible to obtain a uniformly oriented, polarized film and these methods had limits for the development of functional materials exploiting liquid crystal polymers.